The present invention relates to methods for controlled production of surface stabilized particles, such as semiconductor nanoparticles, nanooxides and nanometals (also called nanocrystals or quantum dots) and apparatus for such manufacture.
It is anticipated that the future will be the era of nanotechnology. Through nanotechnology, higher quality products can be made by using smaller amounts of materials to achieve the same desired effects. Customers will receive products at lower costs with greater functionality in smaller packages.
Particles with their smallest dimension between 1 to 100 nm have generated great scientific and commercial interest due to their size-dependent properties and potential uses in electronics, fluorescent imaging, medicine, the chemical industry and everyday life. These size-dependent properties are usually observed at particle sizes below about 20 nm and include the decrease of the material's melting point and a change in the absorbance and/or emission wavelengths depending on the size—quantum size effect. See e.g., Alivisatos, A. P. (1996), “Perspectives on the physical chemistry of semiconductor nanocrystals,” J. Phys. Chem. 100(31): 13226-13239; Eychmuller, A. (2000), “Structure and photophysics of semiconductor nanocrystals,” J. Phys. Chem. B 104(28): 6514-6528; C. B. Murray, C. R. Kagan, M. G. Bawendi (2000), “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies,” Ann. Rev. Mater. Sci. 30: 545-610; M. Green, P. O'Brien (1999), “Recent advances in the preparation of semiconductors as isolated nanometric particles: new routes to quantum dots,” Chem. Commun.: 2235-2241; T. Trindadae, P. O'Brien, N. L. Pickett (2001), “Nanocrystalline semiconductors: synthesis, properties and perspectives,” Chem. Mater. 13: 3843-3858; K. Grieve, P. Mulvaney, F. Grieser (2000), “Synthesis and electronic properties of semiconductor nanoparticles/quantum dots,” Current Opinion Coll. Interface Sci. 5: 168-172; H. Bönnemann, R. M. Richards (2001), “Nanoscopic metal particles—synthetic methods and potential applications,” Eur. J. Inorg. Chem. 2455-22480.
The decreased melting point can be used, for example, to melt nanosilver at low temperatures for flexible printed circuits. In another example, CdSe nanoparticles with sizes from 2 to 6 nm strongly absorb and emit light that ranges in color from blue to red, depending on the size of particles. Nanoparticles that show quantum size effects or size-dependent properties are usually called quantum dots (QDs, q-dots). They can be used for light emitting diodes (LED), lasers, displays, optical devices, as catalysts etc. (see V. L. Colvin, M. C. Schlamp, A. P. Alivisatos (1994), “Light-emitting diodes made from cadmium selenide nanocrystals and a semiconductor polymer,” Nature (London) 370: 354-357); biological fluorescent labels (see M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos (1998), “Semiconductor nanocrystals as fluorescent biological labels,” Science (Washington D.C.) 281: 2013-2016; and W. C. W. Chan, S. Nie (1998), “Quantum dots bioconjugates for ultrasensitive nonisotopic detection,” Science (Washington D.C.) 281: 2016-2018); solar cells (see W. U. Huynh, J. J. Dittmer, A. P. Alivisatos (2002), “Hybrid Nanorod-Polymer Solar Cells,” Science (Washington D.C.) 295: 2425-2427; and W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting, A. P. Alivisatos (2003), “Controlling the morphology of nanocrystal-polymer composites for solar cells,” Adv. Funct. Mater. 13: 73-79); lasers (see V. I. Klimov, A. A. Mikhilovsky, S. Xu, A. Malko, J. A. Hollingsworth, D. W. McBranch, C. A. Leatherdale, H-J. Eisler, M. G. Bawendi (2000), “Optical gain and stimulated emission in nanocrystal quantum dots,” Science (Washington D.C.) 290: 314-317; and H-J. Eisler, V. C. Sundar, M. G. Bawendi, M. Walsh, H. I. Smith, V. I. Klimov (2002), “Color-selective semiconductor nanocrystal laser,” Appl. Phys. Lett. 80: 4614-4616); and catalysts (see T. R Thurston, J. P. Wicoxon (1999), “Photoxidation of organic chemicals catalyzed by nanoscale MoS2,” J. Phys. Chem. B. 103: 11-17).
Nanoparticles are finding widespread applications ranging from composites and coatings to cosmetic creams offering ultraviolet radiation protection. According to a Moore and Michelson report (Woodrow Wilson Int. Center for Scholars, April 2006) currently there are about 212 nanoproducts on the market worldwide. Silver nanoparticles are used as an antibacterial agent in clothing and wound dressing. Nano-oxides are used in the cosmetic industry as sun protection creams, paints, catalysts, in synthetic bones, and as polishing material for silicon wafers. Carbon nanoparticles are used in many commercial products, such as tennis rackets and paint additives. Fluorescent nanoparticles (quantum dots) have commercial applications in bio-labeling, bio-detection, drug delivery and bio-analysis. Future applications of quantum dots include solar cells, lasers, LEDs, flat panel displays, and others. Nanometals can be used as catalysts, in bio-detection, as antibacterial agents and in many other applications.
Currently, colloidal nanoparticles are produced in batch methods on a small scale, which only produces gram-scale quantities, or by using microfluidic reactors which can treat less than 0.3 mL/min of precursor solutions (Barbera-Guillem et al., “Continuous Flow Process for Production of Semiconductor Nanocrystals”, U.S. Pat. No. 6,179,912, Jan. 30, 2001; X. Z. Lin, A. D. Terepka, H. Yang (2004), “Synthesis of Silver Nanoparticles in a Continuous Flow Tubular Microreactor”, Nano Lett., 4:2227-2232; Stott, Nathan E. et al., “Method of Preparing Nanocrystals”, U.S. patent application No. 2005/0112849, May 26, 2005).
The current production limitations for these particles lead to a relatively high price. For example, the current price for fluorescent cadmium selenide quantum dots varies from $2,100 (NN-Labs, LLC) to $20,000/g (Aldrich). Among other weaknesses of these technologies is their inability to generate reproducible results from one batch to another because the reaction rates are rapid and difficult to control at the necessary high temperatures. Thus, each batch of particles will have slightly different sizes and size distribution. Combining particles from different batches will only broaden the size distribution of the product.
Another approach for the synthesis of nanoparticles is a gas phase reaction which produces nanoparticles at high temperatures in the reactor (See book review T. T. Kodas, M. Hampden-Smith, 1999, “Aerosol Processing of Materials”, Wiley: New York, 1999). The time of reaction is short and the cooling rate is high. Thus, obtained particles do not sinter and have a small size from a few nanometers to hundreds of nanometers in diameter. Oxides and nanometals can be obtained by this procedure. The drawbacks of such processes are the broad size distribution, irregular shape, and aggregation of the primary particles.
Synthetic procedures of the invention differ from traditional aerosol processes. In our procedure as described below, we add surfactants to the reaction mixture, and the reaction proceeds in small droplets of a high boiling point solvent. The temperature inside the reactor is lower than the boiling point of the solvent, so the reaction proceeds inside the droplets of aerosol. The mechanism of reactions inside the droplets is similar to the mechanism of batch nanoparticle synthesis reactions in small chemical glassware. The synthesized particles are in colloidal form and soluble in organic solvents. The particles do not agglomerate during storage and can stay in soluble form for a long time. The particle quality is higher relative to size, shape, and size distribution, as compared to the quality of particles made by traditional gas-phase reaction methods.
The present invention overcomes deficiencies in prior U.S. Pat. No. 7,160,489, issued Jan. 9, 2007, which also used chemical aerosol-flow synthesis to synthesize CdSe of nanometer size (Y. T. Didenko, K. S. Suslick, “Controlled Chemical Aerosol Flow Synthesis of Nanometer-Sized Particles and Other Nanometer-Sized Products”, that patent being fully incorporated herein by reference. In this previous work, the mist was created using a household ultrasonic humidifier working at high frequency of 1.7 MHZ, and it was not possible to create a mist from high boiling point solvents alone. The mixture had to be diluted with toluene, a lower boiling point solvent, to ease the transfer of the reaction mixture to the vapor phase. The additional dilution of the precursors solution with low boiling point solvent (toluene) was a prerequisite for mist formation. This dilution decreased the yield and the quality of the quantum dots produced. The size distribution of the quantum dots was broader than desired and a bubbler containing toluene was required to collect nanoparticles produced. Another drawback was that the same solution was sonicated in the same vessel for a long period of time to mist all of the precursors solution mixture. This created the possibility of sonochemically driven reactions in some reaction solutions. Thus, the previously patented aerosol synthesis approach was limited to producing nanoparticles of lower quality, and the production rate of nanoparticles was limited due to low production rate of aerosol from ultrasonic humidifiers.
The size of liquid droplets formed by ultrasonic humidifier used in the previous system (working at 1.7 MHZ ultrasound frequency) was small, about 5 microns. In the current invention, the continuous liquid-flow system produces droplets preferably between 1 and 100 microns in diameter with a most preferable droplet size of about 40 microns.
The positioning of the sprayer directly at one end of the furnace, and preferably on top of the furnace, and the larger size of droplets decrease the residence time of the droplets inside the reactor as compared to the system of the patent. From the patent it was not obvious that reduced residence time would provide a high yield of high quality nanoparticle product. By controlling the size of the furnace (diameter and length), the liquid and the gas flow through the reactor, and the ratio of chemical components in the reaction mixture, it is possible to produce higher quality nanoparticles in a wide range of sizes than the system of the referenced patent.
This present invention provides a new, scalable, and inexpensive method for manufacturing surface stabilized nanoparticles. The nanoparticles are produced using a liquid flow method in which a solution of high boiling point solvent containing precursors flows through a vibrating nozzle, excited by high intensity ultrasound, thus allowing continuous production of droplets. A dense mist of droplets is injected directly into the furnace. The product of the chemical reaction is easily collected at the exit of the reactor, since products (nanoparticles) are dissolved in high boiling point solvent which easily agglomerates or coalesces at room temperature or well above room temperatures. Thus, the advantages of this approach are: a scalable and higher-production process for producing droplets of high boiling point solvent containing nanoparticle precursors and capping agents, a simple method of particle collection, and higher quality nanoparticle product than in previous processes.
This vertical approach, with the sprayer nozzle preferably on top of the furnace, has many advantages, including: full use of precursors from the initial mixture; a flexible production method with the option of changing gas and liquid flow rates independently; a simple collection method (since the nanoparticles are directly confined within the droplets, the droplets and nanoparticles contained within can be directly trapped in a receiving bottle at the output of the reactor); the mixture will not undergo sonochemical reactions in the ultrasonic cell, because sonication only takes place during the short time of atomization; and production yield can easily be scaled up by using commercial spray sources.
The maximum yield of CdSe nanoparticles from previous production system described in the referenced U.S. Pat. No. 7,160,489 was small, about 70 mg/hour. The production apparatus using a continuous liquid flow aerosol production technique described herein allows yields at 15 to 20 times increase in production rate, to about 1 to 1.5 g/hour, and production could be further scaled up. The production rate is only limited by the length and diameter of the reactor tube, i.e. the furnace. The resulting quality of the quantum dots produced is also very good, as indicated by transmission electron microscopy, as well as absorbance and emission measurements. Absorbance and emission are well-known indicators of quantum dot quality. Thus, the quality of quantum dots can be an indication of the performance of the method. The CdSe nanoparticles produced have a fluorescence quantum yield (QY) about 40-50%, with full width at the half maximum (FWHM) about 28-30 nm, and our CdTe q-dots have QY about 40% with FWHM about 30 nm. These are very good numbers and comparable to the best quantum dots published in literature.
The anticipated advances in q-dots applications will require large quantities of inexpensive materials. Currently, the limited number of high-priced nanomaterials products on the market hinders the development of new devices, methods and products. In the future, with improved production methods, the price should drop from thousands of dollars per gram to hundreds of dollars per gram. New applications of quantum dots and new companies based on these applications emerge every day. The demand for nanoparticles is urgent and will accelerate in the next few years. It is imperative to satisfy these demands by creating a technology that will produce high quality quantum dots in the most inexpensive way.